Optical communication glass fibers are made from glass such as quartz glass or doped silica glass and are extremely small in diameter and hence weak in strength. Under normal conditions of use, such fibers may be subjected to considerable bending strain and other tensile forces during the cable manufacturing process and while being pulled through ducts and otherwise bent, twisted, or manipulated during reeling or during installation. In the transmission of light signals through optical fiber cables, the signals are readily attenuated in response to relatively small distortions in the cable, such as those caused by the above stresses, sharp bends or roughness in the surface of the fiber to produce light signal distortions or attenuation at an unacceptable level.
To confine the optical signals inside the signal-transmitting fiber core, a glass or silica fiber core is coated with a glass cladding or an amorphous fluoropolymer coating always of a lower refractive index from that of the optical fiber. The coating on the glass cladding may optionally be a silicone, acrylic, polyimide, or other release agent and a polymer coating, which is usually a hard or soft polymer coating which is coated on the fiber from a melt or a solution of the polymer, or extruded onto the fiber. Many hard and soft plastic coatings have been tried and some of these have been applied in layers for varying purposes as disclosed in U.S. Pat. Nos. 4,113,350, 4,105,284, 4,380,367, 4,072,400, 3,930,103, 4,463,329, 4,099,837, and 4,642,265, of which the background discussion contained therein is hereby incorporated into this application. Another excellent discussion of optical fiber packaging and buffering is provided by a paper in the Bell System Technical Journal, in Volume 54, No. 2, pages 245-262, February 1975, by D. Gloge.
To protect a glass or amorphous fluoropolymer-clad hard polymer-coated glass fiber core, a layer of porous polytetrafluoroethylene (PTFE) tape may be adhered to the hard polymer coating layers by a heat-resistant adhesive coated on the inside of the porous PTFE tape. The soft porous PTFE buffering layer can be covered by jacketing, such as an inner jacket of high-temperature polymer in the form of wrapped laminated tape, a dipped coating from a solvent, or by an extruded melt. A strength member sheath can then be applied followed by an outer jacket of the same polymer materials as the inner jacket or from other materials for environmental protection. This cable is described in U.S. Pat. No. 5,002,359.
It would be desirable, however, to have a cable having greater crush resistance along with higher tensile strength, good flex, and improved thermal isolation of the optical waveguide fiber cable.